This invention relates generally to coaxial dielectric resonators for VHF and UHF bands, and more particularly the present invention relates to such resonators which are small in size, providing high Q.
With the recent tendency of miniaturizing radio equipment for VHF and/or UHF band, intensive research for miniaturizing various parts used in such equipment is being carried on. Especially, filters and resonators used in oscillators are required to have small size to minimize the entire size of radio equipment, such as radio receivers, transmitters or the like.
A well known small resonator for VHF-UHF band, having high Q (low loss) is a quarter wavelength coaxial resonator using TEM mode. This conventional resonator comprises coaxially arranged cylindrical outer and inner conductors and low-loss dielectric filled in the space between the outer and inner conductors so that wavelength reducing effect will be obtained. Namely, the length of such a resonator is shortened as expressed by 1/.sqroot..epsilon.r, wherein .epsilon.r is the specific inductive capacity of the dielectric use.
Although the above-mentioned conventional coaxial resonator has an advantage that it is simple in construction so that it can be readily manufactured, since the resonator has uniform impedance throughout its entire length, resonance points are not only at f.sub.o but also at 3f.sub.o and 5f.sub.o wherein f.sub.o is the fundamental resonance frequency. Therefore, when such a conventional resonator is used in an oscillator or as an output filter of an amplifier, harmonic components of three times and five times the fundamental frequency cannot be removed. In order to solve this problem, therefore, band-pass filters or low-pass filters which remove harmonic components often have to be used together with such a resonator.